Biogas production from algae and sludges: investigating co-digestion, pretreatment, thermophilic digestion and an integrated cycle (HRAP - AD)

Abstract

With population growth and development, the demand for energy is increasing, as well as the need to preserve the environment, given the impacts caused by polluting gas emissions from using non-renewable energy sources. These facts have increased investment in and development of renewable energy sources. Anaerobic digestion is an efficient technology to produce renewable energy, in the form of biogas, from the processing of various types of organic matter, including waste, in addition to the possibility to recover value-added by-products. Microalgae, macroalgae, and waste sludge generated in wastewater treatment plants are promising raw materials for the production of renewable energy through anaerobic digestion. These substrates are generated in ever-increasing quantities and require adequate treatment before being discarded. The anaerobic digestion process is complex, involving four phases, hydrolysis, acidogenesis, acetogenesis, and methanogenesis. Overall, hydrolysis of organic matter is frequently the limiting step in digestion. One way to improve this phase and consequently the digestion process is to perform a pretreatment of the substrate. One of the most used pretreatments is thermal pretreatment. Another way to improve the yield of anaerobic digestion is realizing the anaerobic digestion under thermophilic conditions. To determine the best pretreatment and digestion conditions, a series of batch tests was carried out with microalgae, macroalgae, and sludge. The tested conditions were thermal pretreatment at a temperature of 55 ºC, varying the duration (4, 8, and 12 hours) compared to the anaerobic digestion under thermophilic conditions (55 ºC). It was found that the best digestion condition for microalgae and macroalgae was performing a thermal pretreatment for 4 hours at a temperature of 55 ºC, which resulted in an increase in methane production of 45% and 41%, respectively. For activated sludge, the condition that resulted in the greatest increase in methane production (86%) was digestion under thermophilic conditions. For anaerobic sludge, thermal pretreatment was also investigated, at different temperatures (55 and 95 ºC), substrate/inoculum (S/I) ratio (0.5, 2.0, 4.0, 6.0 and 8.0 on volatile solids basis), and duration of the pretreatment (6, 24, 48 and 72 hours). The best condition, considering the specific methanogenic activity, was an S/I ratio of 6.0 and thermal pretreatment for 48 hours at a temperature of 95 ºC. Like anaerobic digestion, anaerobic co-digestion, in which two or more organic wastes are digested simultaneously, can improve the C/N ratio and nutrient balance, as well as boost biogas production. To verify the efficiency of pretreatment together with co-digestion, between microalgae and activated sludge, six co-digestion conditions were tested, varying the proportions of microalgae (MA) and activated sludge (WAS), presented as MA:WAS ratio, from 0:100 (activated sludge only), 20:80, 40:60, 60:40, 80:20 and 100:0 (MA only). The results showed that 60% of activated sludge can be co-digested with microalgae without harming methane production. Traditionally, domestic wastewater is treated by activated sludge systems or by upflow anaerobic sludge blanket (UASB) systems. However, the activated sludge systems have high costs due to the necessary aeration, while the anaerobic systems are not efficient in removing nutrients, and both systems lack efficiency in removal of pathogens and emerging pollutants. Thus there is a need for the development of new technologies for the treatment of domestic effluents. Microalgae-bacteria systems are a promising alternative for wastewater treatment. Normally, microalgae-bacteria systems have been implemented as single stage High Rate Algal Pond (HRAP) reactors, but a recent variant that aims to improve the productivity and efficiency of the system is the inclusion of an anoxic unit (AX) before the photobioreactor (HRAP), called AX-HRAP. To determine the efficiency of domestic wastewater treatment, the influence of hydraulic retention time (HRT), as well as the presence of emerging pollutants (CECs; compounds of emerging concern) was analyzed in an integrated AX-HRAP system with biomass sedimentation and recycling, coupled with anaerobic digestion of the biomass produced in the system and including biogas upgrading. The HRT in the photobioreactor was initially 4 days (stage I and II), later reduced to 2.5 days (stage III). The integrated system supported high removal efficiencies of organic carbon (98.9 ± 1.1%) and total nitrogen (90.8 ± 8.0%) during the three stages, and with the presence of various contaminants present in the sewage. Phosphorus removal was 68.4 ± 20.1%, 68.3 ± 20.8%, and 53.4 ± 25.0% in stages I, II, and III, respectively. The sedimentation and recirculation of the biomass resulted in better sedimentation characteristics of the microalgal biomass. Finally, the results revealed that domestic wastewater supplementation with CECs exerted a more significant impact on microalgae assemblage than the decrease in HRT.